The conventional practice for determining the propeller forces to which wind tunnel models are subjected, is to use propeller force balances in the form of rotating force measuring devices. These are also known in English as “rotating shaft balances” (RSB) and are positioned between the propeller and a drive shaft. As well as measuring the forces and moments arising in the plane of rotation, this also measures both the thrust and the torque on the propeller shaft induced by the resistance of the propeller, which run vertically in relation to the plane of rotation.
Measuring the thrust of a propeller is a particular feature when measuring propeller forces, because, as the propeller rotates, an air pressure profile is formed in the gap between the propeller hub and the associated drive mechanism or any other fixed element which is detected by the force measuring device as an apparent thrust. In order to calculate this apparent thrust, pressure sensors are positioned on a fixed surface facing the propeller hub, which can calculate the pressure and the pressure distribution in the gap. The apparent thrust can be calculated from the pressure distribution and a knowledge of the surface exposed to this air pressure and then subtracted from the thrust measured by the force measuring device, so that the aerodynamic forces actually created by the propeller can be determined.
In the case of counter-rotating propeller systems with two propellers, there is not just one gap on one side of an individual propeller, but a total of three gaps, in which a corresponding air pressure profile develops. However, calculating pressure distributions in the gaps of such counter-rotating propeller systems is very costly.
DE 33 40 516 A1 and U.S. Pat. No. 7,565,269 B2 disclose methods for correcting total thrust measurements for aircraft engines, in which methods resistances are substracted that depend on the air flow into the engine and through a test cell.
U.S. Pat. No. 4,788,855 A discloses an apparatus for determining the thrust of an engine, comprising a force measuring device and a dynamometer.